Issue: November 15, 1999

Mulches

Question:

What is the best type of mulch to use as a top layer over soil?

Answer:

There is really no "best" mulch to use. The answer depends on the purpose for the mulch and the plants you are growing.

Mulches may be used for several purposes. They may be used to stabilize soil to prevent wind and water erosion. They may be used to reduce the evaporation of water from the soil by creating a protective boundary between the soil and the air as well as shading the soil from the sunlight.

Because the mulching material usually has a greater porosity than the soil, moisture in the soil does not easily move to the air boundary by capillary action, thus evaporation is reduced. By isolating the soil from the air which changes temperature rapidly, mulches maintain more constant soil temperatures, preventing rapid temperature changes from day to night and as the weather suddenly changes. Mulches are often used to reduce weed growth. Annual weeds must germinate from seed each year, but they do not germinate if covered so that they are not in the light. Perennial weeds will grow through most mulches, but may be weakened by the effort to reach the light and by the expenditure of additional food reserves to do so. Finally, mulches can help reduce or delay soil compaction, increasing water and air permeation into the soil. It is also easier to manually remove weeds from non-compacted soils. So, with so many purposes, it is difficult to choose a "best" mulch without knowing the purpose.

There are many different mulching materials. Inorganic mulches, which consist of rock, sand, gravel, cinders, and other natural materials derived from non-living sources are excellent mulches for some purposes. Inorganic mulches serve most purposes for mulches quite well. They reduce erosion effectively, and on slopes larger sizes of organic mulches are superior to most organic mulches because they remain in place and do not float away. Inorganic mulches isolate the soil from drying winds and sunlight, effectively reduce erosion, and help maintain constant soil temperatures. However, inorganic mulches tend to heat the air around them more than organic mulches. They also hold that heat longer into the night to the detriment of plants that respond well to distinct diurnal (day to night) temperature variations. If the inorganic mulch is thick enough, weed problems may be reduced. These mulches can also help prevent soil compaction.

Organic mulches are those derived from once living things, grass clipping, bark, compost, and such. They may reduce erosion but are themselves sometimes subject to erosion by wind because they are light or by water because they float. They can reduce water loss from the soil but, because they are porous and absorb water themselves, are not as effective in this as the inorganic mulches. They can help maintain stable soil temperatures, but in the case of finer particle and decomposed organic mulch, plant roots may grow in the mulch material where they are again exposed to drying and temperature changes. This does not make them inferior to inorganic mulches but does reveal the differences. Organic mulches do not usually become as hot as the inorganic mulches and are not as likely to cause heat injury to plants and may not hold heat as long into the night. Organic mulches decompose releasing nutrients into the soil. This enriches the soil but requires that the organic mulch be replaced periodically. Organic mulches create a good environment for earthworms and, because of the action of the earthworms, soil permeability is enhanced. As organic mulches decompose, they release carbon dioxide into the soil. This can injure some of our arid lands plants which are adapted to well-aerified soils containing high levels of oxygen in the spaces between soil particles. Such plants do poorly when mulched with organic mulches.

Synthetic mulches, those which are manufactured from petroleum products (plastic mulches), are also useful; however, if used improperly, they may cause severe landscape problems. Rolls of plastic mulch are used in field agriculture to reduce herbicide and irrigation requirements. In these cases there are spaces between the plastic rolls. In landscape applications, the plastic mulch is often used over the entire landscape with no gaps through which weeds may grow. The plastic interferes with the infiltration of water and air into the soil. Plant roots require both water and air, so even if there is an irrigation system under the plastic, landscape plants often do poorly under a solid (impermeable) plastic mulch.

To remedy this, porous plastic mulches have been developed. These remedy the problem of water and air infiltration into the soil but often develop problems over time as wind deposits dust and sand above the mulch material. Weeds then germinate in this layer of dust; their roots then penetrate into the soil below through the holes in the plastic. These weeds are difficult to remove manually because their roots are interwoven with the mulch material.

This discussion of mulches will continue with next week's Yard and Garden column.